In a front projection system, the projector and viewer are on the same side of the display surface, with the image from the projector reflecting from the display surface to the viewer. FIG. 1A shows a standard prior art eight-element projection lens 5, which can take an input optical image 2 and project an expanded output optical image 3. FIG. 1B shows a projection light engine 6 that uses the eight element projection lens 5 to project an image directly on a display surface 20. This is an example of an on-axis projection system in which the image is created and projected along a straight axis 10a that is perpendicular to the display surface 20.
For a rear projection system this design must be made more compact to comply with commercial requirements for rear projection screen cabinet dimensions. This “compactness” is quantified in terms of the “throw ratio”. The throw ratio of a projection system is given by the projection distance d (see FIG. 1B) divided by the diagonal length D (not shown) of display surface 20. The diagonal D is measured from the opposite corners of the display surface 20. The throw ratio is given by:
                              Throw          ⁢                                          ⁢          Ratio                =                  d          D                                    (        1        )            
As shown in FIGS. 1C and 1D, the generic projection lens 5 in a front projection configuration produces an image with low distortion, having typical spot sizes (i.e. de-focused point images in the corners). Specifically, with display surface 20 having dimensions of 1480 mm×834 mm, and a 25.4 mm f/2.8 projection lens 5, a projected image is produced with 1 mm spots at the center and 3 mm spots at the corners. A reasonable design goal for a front projection system is to have a minimum resolvable focused spot size of no more than a pixel-span at the center and less than 2-3 pixel-spans in the corners. In this particular example, taking into consideration the pixel size in the micro-display imaging device and the magnification caused by projection onto a distant display surface, a pixel-span at the display surface 20 is about 1 mm. The overall lens distortion is less than 1% over the screen size of the display surface 20. It is desirable for the distortion map (i.e. FIG. 1C) to be rectangular and to exactly overlay the available display area in order to reduce lost pixels and lost brightness (resulting from turning off pixels which do not overlay the display screen and consequently losing their contribution to overall brightness). Furthermore, it is also desirable for the spot diagram of FIG. 1D to show minimal spot size increase (in pixels farther from the center of the display) and to be symmetrical. Minimizing spot size increase implies minimizing point image de-focusing.
Another important design goal in building a compact projection system is to achieve good image quality while minimizing the throw ratio (some designers use the width of the image instead of the diagonal when computing throw ratio, so it is important to specify which definition of throw ratio is being used). Minimizing the throw ratio is especially important for rear projection systems in which the projector and screen are physically combined into a single functional unit, such as rear projection televisions. In such units minimizing the throw ratio implies a shorter projection path length, allowing for a smaller depth for the cabinet, which houses the display surface and the projection light engine. Minimizing the throw ratio in front projection systems allows large images to be projected with a projector placed very close to the screen.
Cabinet depths and depth reductions in rear-projection display systems are evaluated objectively by measuring the ratio of display diagonal to cabinet depth or DtoD ratio. Conventional configurations using on-axis projection, flat mirrors, and optical-only means of distortion correction have yielded DtoD ratios of about 2.5 to 3.5 (for example, a 61″ diagonal with a 19.5″ depth, or a 55″ diagonal with a 18″ depth, etc).
To decrease cabinet depth, prior art methods have combined flat mirrors (to fold the optical path) with low distortion and wide field of view (FOV) lenses which serve to decrease the projection path length, hence decreasing the throw ratio. By fine-tuning the optical geometry (lens type, focal distances, mirror angles), image distortions can be minimized. Prior art configurations place the fold mirror (or mirrors) on-axis to the projected beam. This has the advantage of not producing keystone distortion. However, the disadvantage is not providing significant cabinet depth reductions, or not increasing in the DtoD ratio.
For instance, FIG. 2A, shows a prior art projection system 6′ with a projection light engine 14, a planar mirror 8, and display surface 20. This is an example of an on-axis projection system in which the planar mirror creates a folded optical path and lies at an angle α to the display surface 20. The result is a cabinet depth of T. This projection system 6′ does not result in keystone distortion as can be seen in FIG. 2B which shows an image I projected on the display surface 20.
The main method for reducing cabinet depth in prior art configurations is to use short-throw, wide-angle lenses with an on-axis optical path. This has the disadvantage of limiting depth reductions, and even though keystone distortion is not present, this approach still requires optical elements that are challenging to design and manufacture. The optical and geometric constraints manifest themselves as increased pincushion or barrel distortion and keystone distortion. The design of prior art systems has largely been constrained by the requirement of minimizing these distortions along with achieving a required Modulation Transfer Function (MTF), correcting for lateral color, meeting lens F-number specifications, while satisfying cost-performance tradeoffs.
Prior art rear projection systems use screen assemblies that have low reflectance to light impinging on them from the front (by use of light absorbing materials) in order to provide a high contrast ratio. These screen assemblies also have a high transmittance for light impinging on them from the rear (by use of lenticular arrays and collimation of light) in order to provide high brightness. Light is typically collimated by using a Fresnel lens as part of the screen assembly. A Fresnel lens is a symmetrical circular structure (its optical center is located at the physical center, or on the axis of the projection light path) for on-axis projection systems. A Fresnel lens of a given focal length substitutes for a large circular biconvex lens of the same focal length. The diameter of such a Fresnel lens is at most the length of the display diagonal. These Fresnel lenses are typically thin, very flexible and expand with interior temperature rise. The image quality for on-axis projection systems is not very sensitive to variations in the central portion (around the optical axis) of a Fresnel lens' surface profile. Unfortunately, these symmetrical Fresnel lenses cannot be used in off-axis RP systems. In addition, the Fresnel lens must be carefully designed in projection systems with high DtoD ratios because the light impinges on the rear of the screen at incident angles that vary from a minimum near the bottom of the screen of from 20±5 degrees to a maximum of up to 60±5 degrees near the top of the screen. Accordingly, the lens surface must be maintained very precisely because of the sensitivity of the collimation function to the angle of incidence of the impinging light.